Liquid crystal materials are useful for electronic displays because light traveling through a layer of liquid crystal (LC) material is affected by the anisotropic or birefringent value (.DELTA.N) of the material, which in turn can be controlled by the application of a voltage across the LC material. Liquid crystal displays are commonly used in applications such as digital watches, calculators, portable computers, avionic cockpit displays, and many other types of electronic devices which utilize the liquid crystal advantages of long life and low power consumption.
Gray level performance and the corresponding amount of inversion are important characteristics in determining the quality of a liquid crystal display (LCD). Conventional LCDs typically utilize anywhere from about eight to sixty-four different driving voltages. These different driving voltages are generally referred to as "gray level" voltages. The intensity or shade of light transmitted through the pixel or display depends upon the driving voltage. Accordingly, gray level voltages are used to generate dissimilar shades of color so as to create different colors when, for example, these shades are mixed with one another.
Preferably, in a normally white (NW) twisted nematic (TN) liquid crystal display, the higher the driving voltage, the lower the intensity (fL) of light transmitted through the display panel. Thus, the lower the driving voltage, the higher the intensity of light reaching the viewer in preferable circumstances. The opposite is true in normally black (NB) twisted nematic displays.
By utilizing multiple gray level driving voltages, one can manipulate, for example, normally white and normally black twisted nematic LCDs to emit desired intensities and shades of color. A gray level voltage is generally known as any driving voltage greater than V.sub.th (threshold voltage) up to about 5.0-6.5 volts. An exemplary V.sub.th is about 1.0 volt.
In normally white TN displays, it is desirable to have a transmission vs. driving voltage curve wherein the amount of light reaching the viewer continually and monotonically decreases as the driving voltage increases. In other words, it is desirable to have gray level performance in a NW display such that the transmission at 6.0 volts is less than that at 5.0 volts, which in turn is less than that at 4.0 volts, which is less than that at 3.0 volts, which is less than that at 2 volts, etc. Such good gray level curves across a wide range of viewing angles allows for the intensity of light reaching the viewer to be easily and consistently controlled by the gray level voltages thereby permitting the viewer to see the same image from all viewing angles.
FIGS. 1-2 are computer simulation transmission vs. driving voltage graphs of a prior art TN normally white liquid crystal display which is not provided with any retarders. FIG. 1 plots transmission vs. driving voltage for a plurality of horizontal viewing angles aligned along the 0.degree. vertical viewing axis while FIG. 2 plots transmission vs. driving voltage for a plurality of vertical viewing angles all aligned along the 0.degree. horizontal viewing axis. The prior art normally white LCD of FIGS. 1-2 had a cell gap of 5.50 .mu.m, a pretilt angle of about 3.degree., and front and rear linear polarizers whose transmission axes were arranged at an angle of about 90.degree. with respect to one another. Light having a wavelength of about 550 nm was utilized in plotting the graphs of FIGS. 1-2.
The purpose of prior art FIGS. 1-2 is to illustrate the fact that when no retarders or retardation films are provided, a normally white TN LCD experiences significant inversion problems at certain horizontal viewing angles. The vertical angles along the 0.degree. horizontal axis plotted in FIG. 2 do not experience significant inversion problems as illustrated by the fact that each transmission vs. driving voltage curve in FIG. 2 when gray level voltages are applied decreases continually and monotonically for the most part as the driving voltage increases. However, the horizontal viewing angles plotted in prior art FIG. 1 experience significant inversion problems as evident by the fact that at least the .+-.60.degree. horizontal viewing angle curves and the .+-.45.degree. horizontal viewing angle curves increase a substantial amount with respect to transmission % as the driving voltage rises past about 3.0 volts.
In other words, the horizontal viewing angles of .+-.45.degree. and .+-.60.degree. along the 0.degree. vertical viewing axis plotted in FIG. 1 experience undesirable inversion. Unlike some of the other plotted viewing angles, these four curves in the gray level range do not continually and monotonically decrease as the driving voltage increases. Contrary to this, they actually increase as the driving voltage rises above about 3.0 volts thereby creating what is known in the art as an "inversion hump". The inversion humps illustrated in FIG. 1 include only rise portions. However, such inversion humps often include both rise and fall portions as will be appreciated by those of skill in the art thus enabling the "inversion humps" to actually look like humps.
An ideal transmission vs. driving voltage curve for a NW display has a decreased transmission for each increase in gray level driving voltage at all viewing angles. The curves of FIG. 2, while not perfect, are examples of satisfactory transmission vs. driving voltage curves with respect to inversion. humps.
In contrast to this, the inversion humps shown in FIG. 1 of at least the .+-.45.degree. and .+-.60.degree. horizontal angles represent increases in transmission for corresponding increases in gray level driving voltages above about 3 volts. As will be recognized by those of skill in the art, the illustrated inversion humps create problems because while transmission at certain viewing angles is either decreasing or remaining steady, transmission at the problematic angles of .+-.45.degree. and .+-.60.degree. horizontal is rising substantially thereby causing different images to be portrayed to the viewer at different viewing angles.
While the prior art normally white TN LCD of FIGS. 1-2 void of retarders experiences inversion problems at certain horizontal angles, another prior art normally white TN LCD (including retarders) is portrayed in computer simulation FIGS. 3-4. This LCD suffers from inversion not at horizontal viewing angles, but at the vertical viewing angles of +30.degree. and +40.degree.. Thus, while the FIGS. 1-2 LCD had inversion problems in both the positive and negative horizontal regions, the FIGS. 3-4 LCD experiences inversion only in the positive vertical viewing region.
The prior art normally white LCD plotted in prior art FIGS. 3-4 had a cell gap of 5.50 .mu.m, a pretilt angle of about 3.degree., front and rear linear polarizers whose transmission axes were aligned about 90.degree. from one another to define a normally white TN cell, and a pair of negatively birefringent retardation films each having a retardation value of about -180 nm. In this NW display, the first retarder was disposed on the front side of the LC layer between the glass substrate and the front polarizer and the second retarder was located on the rear side of the LC layer between the rear glass substrate and the rear polarizer. Light having a wavelength of about 550 nm was utilized in the FIGS. 3-4 computer simulations.
As shown in FIG. 3, this normally white LCD did not experience substantial inversion at the horizontal viewing angles plotted along the 0.degree. vertical axis. Unfortunately, this display did suffer from inversion at the plotted vertical viewing angles of +30.degree. and +40.degree. shown in FIG. 4. The inversion humps for these particular vertical viewing angles cause the viewer to see different images at these angles than at the other plotted viewing angles when gray level voltages of from about 2.0 to 6.0 volts are utilized. This, of course, is undesirable.
As can be seen from prior art FIGS. 1-4, a typical normally white LCD without retarders experiences inversion problems in both the positive and negative horizontal viewing regions. While it may be difficult to see, the +60.degree. and -60.degree. horizontal curves in FIG. 1 overlap one another. The same is true for the +45.degree. and -45.degree. curves as well as the remaining horizontal curves in FIG. 1. This is also the case in all other horizontal (not vertical) transmission versus driving voltage graphs herein.
However, when a pair of negatively birefringent retarders are provided to the display, the inversion problem shifts from the positive and negative horizontal regions (FIG. 1) to the positive vertical region (FIG. 4). Thus, while the addition of the retarders eliminates the problem of inversion in the horizontal viewing regions, it creates a similar problem in the positive vertical viewing region.
It is apparent from the above that there exists a need in the art for a liquid crystal display which can substantially eliminate the above discussed problems of inversion while still providing the contrast ratio benefits given by retardation films. Such a display would, of course, have improved gray level viewing characteristics and a better overall appearance to the viewer.
The computer simulations of normally white LCDs set forth herein each included a liquid crystal material with a birefringence (.DELTA.N) of 0.084 at room temperature, such as that of Model No. ZLI-4718 commercially available from Merck.
The term "retardation value" as used herein means "d.multidot..DELTA.N" of the retardation film or plate, where "d" is the film thickness and ".DELTA.N" is the film birefringence (either positive or negative). The retardation value can be positive or negative depending upon the value of the film birefringence.